1. Field of the Invention
This invention relates generally to gas flow and determination of gas flow rates. This invention relates more specifically to dryers and to gas flow rate determination for such dryers used to dry granular resin and other granular and powdery materials, and particularly relates to air blowers and air flow rate measuring devices used with such dryers.
2. Description of the Prior Art
Dryers for granular resin material and other powdery materials fall into two categories: vacuum dryers and desiccant-based dryers. This invention pertains principally to desiccant-based dryers, but also has applicability to vacuum dryers. Exemplary desiccant-based dryers are disclosed in U.S. Pat. Nos. 5,487,225 and 7,007,402; exemplary vacuum dyers are disclosed in U.S. Pat. Nos. 6,154,980 and 7,234,247.
Dryers, specifically desiccant-based dryers, use blowers to move air into hoppers filled with granular material to be dried by the dryer. Dryer users, knowing the rated cubic feet per minute of air flow capability of the dryer blower, often want to know the cubic feet per minute of air the blower is actually moving. The number of cubic feet per minute of air moved by the blower reveals the condition of the air filter in the system, and may reveal or affect other parameters of interest.
As a result, dryer users many times install devices for measuring air flow in cubic feet per minute with these devices positioned in conduit carrying an air stream into or out of the dryer hopper. These known air flow rate measuring devices are notoriously inaccurate and are also sensitive to heat. The air stream may still be warm if it has been used to dry granular resin or other granular or powdery material of interest by heating that material to drive moisture from the material. As a result, known devices used to measure flow rate of air passing through an air duct may be essentially useless if the temperature of the air flowing therethrough is excessive.
From the foregoing, it is apparent that there is a need for more accurate, and desirably less expensive, air flow rate measuring technology.